The present invention relates to subscription television systems and more particularly to methods and apparatus for implementing a high security pay television system wherein real time control of each subscriber's control unit is exercised by the central transmitting station.
It is well known in the subscription television field to provide means for insuring that only authorized subscribers may gain useful access to transmitted programming materials. Since its inception, the pay television industry has employed a wide range of techniques to assure that the proper revenue is derived in payment for programs provided. Initially, periodic flat rate charges were levied on all subscribers connected to a particular network, and the subscribers were then provided with a straightforward conversion device enabling reception of all programming transmitted. As the industry developed it became obvious that the early systems were too susceptible to abuse by unauthorized use of the converters, and equally importantly did not provide the degree of control required to differentiate between premium, higher cost, programs and more routine subject matter. To meet the new requirements imposed by the subscribers' desire to receive a wider mix of programs, and to assure proper pro rata revenue on a per-program basis, a number of more selective equipment devices evolved. The devices afforded the individual subscriber a wide choice of programs and provided the means whereby the CATV operator could exercise an increased degree of control against the unauthorized use of the programming.
Illustrative of prior art systems wherein a moderate degree of security is employed, is U.S. Pat. No. 3,885,089 to Callais et al which describes a system wherein scrambling of television programs is achieved by transmitting in an alternating sequence a pair of television programs on a pair of channels. The receiving stations then assemble a complete program by deinterleaving the transmitted waveforms. An exemplary disclosure of a prior art effort to exert positive control of the channel frequency in use at the subscriber's location is shown in the U.S. Pat. No. 3,914,534 to Forbes. Forbes teaches the use of control signals originating at the central station to enable coding means already resident in the subscriber's terminal to determine the channel frequency to be enabled. Other illustrative prior art is found in the U.S. Pat. No. 3,852,519 to Court, and in U.S. Pat. No. 3,919,462 to Hartung et al.